In currently used systems of telephony, each subscriber has a telephone set which includes a microphone capable of converting an acoustic signal into an electric signal and an earphone capable of converting an electric signal into an acoustic signal. The various telephone sets are connected by electric wires to an electric switching centre. Each telephone set includes selection means, e.g. of the well-known rotary dial type, so that each subscriber can use his telephone set to generate a signal which is characteristic of another subscriber with whom he wishes to communicate. This signal is transmitted to the switching centre which includes e.g. a ringing system to alert the called subscriber so that the calling and called subscribers can communicate with each other.
Since the discovery of low loss optical fibres and of semiconductor lasers operating at ambient temperature, tests have been made using optical channels for transmitting data from each subscriber telephone set to a switching centre. To transmit data, each telephone set includes a semiconductor laser which emits a light wave at one end of an optical "send" fibre whose other end is connected to the switching centre. Each telephone set further includes means for modulating the amplitude of the emitted light wave by the frequency of the electric signal delivered by the microphone as well as means for demodulating the optical signal transmitted to the telephone set from the switching centre via an optical "receive" fibre. The optical send and receive fibres of each telephone set are connected at their remote ends to the switching centre via electro-optical transducers which convert the modulated optical signals into electric signals and vice-versa. The switching centre remains absolutely identical to electric switching centres currently used in electric telephony systems.
The disadvantage of the above-described system of optical telephony is that its electric switching centre is complex and expensive and that it requires electro-optical transducers for each fibre connected to the switching centre.
Preferred embodiments of the present invention mitigate this disadvantage and produce a particularly simple system of optical telephony in which switching is also performed by optical means.